Frequency modulation detector and converter



June 19, 1945. R ALBRIGHT 2,378,819

FREQUENCY MODULATION DETECTOR AND CONVERTER .F'iled Nov. 24, 1942 2 Sheets-Sheet 1 01/7/ 117 4/0 mpur (G June 19, 1945.

R, B. ALBRIGHT FREQUENCY MODULATION DETECTOR AND CONVERTER 2 Shets-Sh set 2 Filed Nm 24, 1942 Patented June 19, 1945 FREQUENCY MODULATION DETECTOR AND CONVERTER Robert B. Albright, Philadelphia, Pa., assignor to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application November 24, 1942, Serial No. 466,763

10 Claims.

This invention relates to frequency modulation systems, and more particularly to a novel circuit for detecting frequency-modulated carrier signals, and to means, including said detector circuit, for converting such signals to amplitude-modulated carrier signals having no substantial frequency modulation components.

The invention is applicable to frequency modulation systems generally, and it is readily adaptable to narrow-band frequency modulation systems, wherein the maximum frequency deviation may be of the order of a few kilocy cles or less. Under these circumstances a frequency modulation detector of unusually high sensitivity is desirable in order to provide a satisfactory detected output from the relatively small deviations of the carrier wave. Aside from its evident use in frequency modulation communication channels, the circuit of the present invention is adaptable to various special applications of. which one will be described in detail.

The principal object of the invention is to provide a novel frequency modulation detector employing a bridge network as a frequency-discriminator to obtain an amplitude modulation component corresponding to the frequency modulation of the signal to be detected.

Another object of the invention is to provide a. frequency modulation detector of this character in which the bridge network is operated over a linear frequency range on one side of the null frequency (balance) point of the bridge.

; A further object of the invention is to provide a frequency modulation detector employing either a bridged T network or a parallel T network for the stated purpose.

Another object of the invention is to provide a frequency modulation detector of simple lowcost construction.

Still another object of the invention is to provide a frequency modulation detector employing a frequency discriminating network which possesses a very steep frequency response characteristic over a specified range, thereby to provide high detector-sensitivity within said range.

The invention itself, as well as other of the objects thereof, will be understood by those skilled in the art from a consideration of the following description and accompanying drawings, in which:

Fig. 1 is a schematic illustration of a circuit utilizing the invention;

Fig. 2 shows a typical frequency response characteristic for the frequency discriminating network employed in the present invention;

Fig. 3 illustrates a modified portion of the cir' cuit ofFig. 1; q

Fig. 4 is a schematic illustration of a detector circuit embodying. the invention;

Fig. 5 illustrates an alternative form of the bridge network employed;

Fig. 6 is a schematic illustration of a pushpull form of the invention; and

Fig. 7 shows the frequency response characteristics for the two frequency discriminating networks employed in theembodiment of Fig. 6.

Referring to Fig. 1 of the drawings, the frequency modulation detector per so, as it constitute's a part of the "invention, comprises the frequency discriminating network I, the gridcondenser grid-leak combination 2-3, and the input elements (1. e. the grid 4 and cathode 5) of the vacuum tube 6. The frequency discriminating network I consists of a pair of series condensers each of value awfci A typical frequency response characteristic of the network I is illustrated by the full-line curve A in Fig. 2. In a bridged T network of this character, wherein the shunt resistor has a resistance equal to four times the resistance R of the coil L, the ratio of output to input voltage (i. e. gain.) at the resonant frequency is substantially zero. If, as is preferred, a coil L of relatively high Q is employed so that its resistance R is small, the response of the network will rise steeply and linearlyover a substantial range before flattening out. Consequently, if a frequency-modulated carrier signal is applied to the input terminals of the network I, and the resonant frequency of the network is adjusted so thatthe frequency deviations of the carrier occupy the linear portion of the network's .chaiacteristic, the frequency-modulated signal as derived from the output terminals of the network will be strongly amplitude modulated. The wave at this point is a hybrid signal having both amplitude and frequency modulation components. This hybrid signal may be supplied to any suitable rectifier circuit such as a diode rectifier,

or, as in this instance, a rectifier whichcomprises the cathode 5 and input grid 4 of the tube 6, together with the associated grid condenser and grid leak 3. The detected audio signal, which is derived from the amplitude modulation of the hybrid wave, is present at the input grid 4 and may be utilized directly, or employed to effect other desired functions, an example of which use will be described presently. If the audio signal is to be used directly, as in a frequency modulation receiver, it would be preferred to employ, in place of the grid-leak form of detector, the more conventional diode type of detector. This will be described hereinafter with particular reference to Fig. 4.

" The quality of the bridged T type of network which makes its use in a frequency modulation detector highly advantageous is the unusual steepness of its frequency response characteristic in the region of resonance. The characteristic A shown in'Fig. 2, based on actual measurements or a network embodied in a working model of the invention, illustrates how, within the space of four kiiocycles, the response of the network changes linearly from approximately zero (at resonance) to 65 per cent before reaching the curved region of the characteristic. Since the linear portion of this characteristic extends over a frequency range of approximately four kilooscillator tube I and its associated resonant circuit. 8. Although the oscillator stage may be regarded broadly as any source of frequency-modulated signals, for purposes of illustration there is shown an oscillator whose frequency is varied by means of a frequency modulation phonograph pick-up of the type d'e-' scribed in my co-pending application Serial No. 397,986, filed June 13, 1941-. The coil 9, which forms a part of the resonant circuit 8, may con-- sist of a small inductor whose inductance is adapted to be varied through the agency of a stylus indicated diagrammatically by the reference numeral Ill. The frequency modulated output of the oscillator may conveniently be derived from the anode I I of the tube I, the anode load l2 being connected, as shown for that purpose.

If, for example, the resonant frequency of the frequency discriminating network I is adjusted to 455 k. c., and if the linear portion of the characteristic extends to four kilocycles on either side of resonance (e. g., as indicated in Fig. 2), the resonant frequency of the circuit 8 may most advantageously be adjusted either to 453 k. c. or 457 k. c., thus permitting a maximum 'fre quency deviation of two k. c; on either side of the unmodulated carrier frequency while still recycles the optimum operating point is at two kilocycles either side of resonance. A network having the characteristic shown in Fig. 2 will perform most satisfactorily for frequencyv modulated signals in which the deviation on either side of the +2 k. 0. operating point does not exceed two kilocycles.

A comparison between the frequency response characteristic of the bridged T network and that of a simple series resonant circuit employing elements of comparable Q (ratio of reactance to resistance), is afforded by Fig. 2, in which the dashed-line curve B represents the characteristic of the latter circuit. It is immediately evident that curve B does not possess the long straight working range of the characteristic A, and it follows that from the standpoint of distortionfree operation the characteristic A is the preferred one. It is likewise evident that the frequency sensitivity of the bridged T network greatly exceeds that of the series resonant circuit, as is shown by the steepness of the slope of curve A as compared to the slope of curve B. Since overall detector sensitivity is directly proportional to the frequency sensitivity of the frequency discriminating network, the superiority of the bridged T system is apparent.

While the bridged T network of the present invention provides improved performance over that obtainable from a simple series resonant circuit even when the bridging resistor is not precisely equal to four times the resistance R of the coil L, it will be understood that the response of the network at resonance falls to zero only when this preferred relationship is adhered to, and it is only under these conditions that the maximum linearity and steepness of the curve is obtained. In this regard it may be stated that the higher the Q of the inductance coil L, the steeper will be the curve A, and, hence, the greater the sensitivity of the detector.

As employed in Fig. 1, the improved frequency modulation detector is utilized as an element in a system for converting a frequency-modulated carrier signal of one frequency into an amplitudemodulated carriensignal of substantially difierent frequency. Referring again to Fig. 1, there is shown a first oscillator stage comprising the maining on the linear portion of the networks frequency response characteristic.

In order to convert the frequency-modulated signals supplied by the oscillator 1 to a strictly amplitude-modulated.signal (as contrasted to the usual hybrid signal) a second oscillator circuit is required. The second oscillator may comprise a resonant circuit It in association with the oscillator elements 5, l4 and 15 of the multi-grid vacuum tube 6. In ,order to prevent undesired interaction between the two oscillators, the frequency of the second oscillator is preferably selected so that the harmonics of one oscillator do not correspond with the fundamental or harmonies of the other oscillator.v In the particular example under consideration, the frequency of the second oscillator was adjusted to 1500 k. c.

As has already been indicated, in the operation of the circuit a detected audio frequency signal is present at the control grid 4 of the tube 6. Since this grid is interposed in the space current path of the oscillator, it is evident that the carrier frequency oscillations generated in the tube 6 will be amplitude-modulated by the audio signal voltage present on the grid 4. In this manner very sub stantial degrees of modulation may be secured without the need -for complicated modulation signal being converted to a 1500 k. c. amplitudemodulated signal with 30% modulation, the latter signal being supplied by way of the output winding IE to a small antenna (not shown) and thence to the receiving antenna of a conventional broadcast band radio receiver. The electrical constants of the frequency discriminating network -I were as follows: L=1300 microhenries, C=47.5 micromicrofarads, and R=35 ohms. The resonant frequency of the bridged T network was approximately 455 k. c.

The circuit of Fig. 1 can also be employed to effect amplitude modulation of a carrier wave Without the necessity of passing through the frequency modulation step hereinbefore described, In this modification of Fig. 1, shown in Fig. 3, a carrier wave source I! is employed to generate a comprisethe oscillator 1-8 of ductance' remaining constant.

stant, but since the intermediate frequency f the network I may be brought .of series resistors and. network can be balanced to give zerogain at a predetermined frequency as follows. condensers of the first T section should beof like This source may Fig. l'with its in- Theinductanc'e L a of the frequency discriminating network I is re- 1 placed by a phonograph pick-*up element empioying anlinductance unit of the type alreadydescribed with reference to coil 9 of Fig. -1, the

phonograph stylus being diagrammatically indicated by the arrow l8. In this embodiment of the invention; the frequency of the signal applied to the input terminalsof the network I is conresonant frequency of the network I is made to 'vary, it follows that the carriersignal as derived from the output terminals of the network I will be amplitude-modulatedin precisely the same manner that the fresignal was amplitude-mod- Reference has already been made to the use of the'present invention in the detector stage of a conventional frequency modulation radio receiver. A circuit suitable for this purpose is illustrated in Fig. 4. The generalizedsource l9 of frequency modulated signals will ordinarily comprise the output stage of the receivers intermediate frequency amplifier. To this source is connected the frequency discriminating circuit I, already described with reference to Fig. 2. The signal rectifier in this instance consists of the diode D, the coupling condenser 2,. and the diode load resistor 3. The detected be supplied to a suitable output circuit, e. g., the

output signal may volume control potentiometer 20, by way of an ilter network 2 l22, and

the coupling condenser 23 l In wide band systems, the frequency discriminating network .I is preferably designed to resonate at'a higher frequency (e. g., 4.3:;mc.) in

order that the network may present asubstantially linear frequency characteristic over the wide deviation range of the signal to be received. In addition, it may be necessary to employ "resonant elements of lower Q than would be utilized in a narrow band system. The inductor L imay conveniently be provided with a movable comminutediron core member 24, by means of which to resonance at any predetermined frequency. v

In Fig. 5, there is shown another form of bridge network which may be employed as the frequencydiscriminating device. This device is a parallel T network and comprises a pair of 'T sections having their input and output terminals connected in parallel relation. One of the T sections comprises a pair of series condensers and a shunt resistor, while the othersection comprises a pair a shunt condenser. This The series capacity, and the capacity of each should be equal to one-half the capacity of the shunt condenser of the second T section. The series resistors of the second T section should be of like resistance, and the resistance of each should be equal to twice the resistance of the shunt resistor of the first T section. Finally, it is necessary that I where f is the null frequency (i. e. the frequency at which the gain of the network is zero), R. is the resistance of the shunt element of the first T tapped secondary Winding. a D1 and D2 are provided with and 21 each shunted by the condensers 28 and i 21 are connected to each other steep frequency system. i

work does not require the In the use of this bridge network according to the invention, the operation takes place on one side of the null frequency point, as previously tie scribed. While this network does not exhibit the characteristic of the bridged 1' network, it is well adapted for use in a-irequency modulation detector, particularly ina wide band It is important to note that the parallel T netuse of inductance" coils, but utilizes resistors and condensers related in a very simple manner. Hence, it is capable of easy and economic construction. a

In a physical embodiment of this device, the electrical constants were as follows: micromicrofarads and R=2000 ohms. The null or balance frequency of the network was 398 k. c. Reference is noW made to Fig. 6, in which there is illustrated a push-pull form of the detector circuit of Fig. 4. In the embodiment of Fig. 6 a pair of single-sided detector circuits (of the type illustratedin Fig. 4) are connected backto-back to form a push-pull or balanced type of detector. While the push-pull form of the invention is not limited to the use of any particular type of single-sided bridge network, for purposes of illustration a pair of bridged T networks la and lb have been shown. These networksare connected in push-pull relation to the source of frequency-modulated signals l9 through the agency of a transformer 25 having a center.-

The detector diodes load resistors 26 29 respectively. The diode load resistors 26 and circuit 23, 20 in balanced relation.

Reference may be had to Fig. 7 for an illustration of the preferred mode of operation of the circuit of Fig. 6. Assuming quency (the frequency of the'undeviated "frequency-modulated carrier) of the signal to be received is ,fc, the bridge networks la and, lb are preferably balanced for zero transmission at the frequencies f1 and f2 respectively, said frequencies being respectively below and above thesaid center-frequency. The frequency characteristic of thebridged T network la is illustrated by the ment of the invention similarly designated curve of Fig. 7, while the frequency characteristic of the bridged T network lb is designated lb. In the preferred embodiment the crossover point 30 between the two frequency characteristic occurs at the frequency fc, and at the mid points of the linear portions of the characteristics. The push-pull embodihas the advantage that distortion resulting from nonlinearity of the networks is greatly minimized.

Although the invention has been described and illustrated with particular reference to certain preferred embodiments, it should be understood that numerous alterations and modifications may be made within the scope of the invention, as defined in the appended claims.

I claim:

1. In a phonograph record player, the combi nation comprising a first vacuum tube oscillator, a phonograph pickup device constructed and arranged to frequency-modulate the signal generated by said oscillator, a bridge networkarranged to receive the frequency-modulated signal and adapted to function as afrequency discriminator,

a second vacuum tube oscillator constructed and arranged to generate a carrier wave of substantially fixed frequency, and means responsive to a and to the output I that the center-frefixed frequency, a resonant signal derived from said bridge network for emplitude-modulating the carrier wave derived from said second vacuum tube oscillator.

2. In a phonograph record player, the combination comprising a source of carrier signal of bridged T network connected to said source, a phonograph pickup device of the variable reactance type, said device constituting an element of said network, the resonant frequency of said network being varied at an audio frequency rate by the operation of said pickup, whereby the gain of said network, and hence the amplitude of the transferred carrier signal, is varied in substantial accordance with the reactance variations of said pickup,

3. In an electrical system, a source of frequency modulated signals, a first bridged T network balanced for substantially zero transmission at a predetermined frequency, a first rectifier means connected to the output terminals of said network, a second bridged T network balanced for substantially zero transmission at a different predetermined frequency, a second rectifier means connected to the output terminals of said second network, means for applying signals from said source to said bridged T networks in push-pull relation, and means connecting at least a portion of said rectifier means in balanced relation.

4. In a frequency-modulation detector, at frequency discriminating circuit comprising a. bridged T network and rectifier means connected thereto, the constants of said network being so selected that its gain is substantially zero at one frequency, the gain varying substantially linearly from that frequency over a predetermined operating range, said range including the frequency range of the frequency-modulated signal to be detected.

5. In a frequency-modulation detector, a frequency discriminating circuit comprising a bridged T network whereof the upright of the T is an inductance coil, the arms of the T are condensers andthe bridging element is a resistor nation comprising a first oscillator, a phonograph whose resistance is substantially four times the resistance-of the inductance coil, the constants of said network being so selected that its gain is substantially zero at one frequency, the gain varying substantially linearly from that frequency over a predetermined operating range which includes the frequency range of the frequency-modulated signal to be detected, and rectifier means connected to said network.

6. In a frequency modulation detector, a bridged T network adjusted to operate as a frequency discriminator over the frequency range of the signal to be detected, and a signal rectifler connected to the output terminals of said network, said network being characterized in that the arms of the T are capacitive, the upright element of the .T is resistive and inductive, and the bridging element is resistive and is bridged across the capacitive arms.

7. A frequency modulation detector as claimed in claim 6, wherein the resistance of the bridging element is substantially four times the resistance of said upright element.

8. In a frequency .modulation detector, a bridged T network adjusted to operate as a frequency discriminator over the frequency range of the signal to be detected, and a signal rectifier connected to the output terminals of said network, said network comprising a pair of series reactance elements constituting the arms of the T, a reactance element of different character in the upright of the T, and a resistance element bridgedacross said series reactance elements.

9. In a phonograph record player, the combipickup device constructed and arranged to frequency-modulate the signal generated by said oscillator, a frequency discriminator device arranged to receive the frequency-modulated signal, a second oscillator constructed and arranged to generate a carrier wave of substantially fixed frequency, and means responsive to a signal derived from said discriminator device for amplitude-modulating the carrier wave derived from said second oscillator.

10. In a phonograph record player, the combination comprising a source of carrier signal of fixed frequency, a resonant bridge network connected to said source, a phonograph pickup device of the variable reactance type, said device constituting an element of said network, the resonant frequenc of said network being varied at an audio frequency rate by the operation of said pickup, whereby the gain of said network, and hence the amplitude of the transferred carrier the reactance variations of said pickup.

ROBERT B. ALBRIGHT. 

